W18O49 Nanowhiskers Decorating SiO2 Nanofibers: Lessons from In Situ SEM/TEM Growth to Large Scale Synthesis and Fundamental Structural Understanding

Tungsten suboxide W18O49 nanowhiskers are a material of great interest due to their potential high-end applications in electronics, near-infrared light shielding, catalysis, and gas sensing. The present study introduces three main approaches for the fundamental understanding of W18O49 nanowhisker growth and structure. First, W18O49 nanowhiskers were grown from γ-WO3/a-SiO2 nanofibers in situ in a scanning electron microscope (SEM) utilizing a specially designed microreactor (μReactor). It was found that irradiation by the electron beam slows the growth kinetics of the W18O49 nanowhisker, markedly. Following this, an in situ TEM study led to some new fundamental understanding of the growth mode of the crystal shear planes in the W18O49 nanowhisker and the formation of a domain (bundle) structure. High-resolution scanning transmission electron microscopy analysis of a cross-sectioned W18O49 nanowhisker revealed the well-documented pentagonal Magnéli columns and hexagonal channel characteristics for this phase. Furthermore, a highly crystalline and oriented domain structure and previously unreported mixed structural arrangement of tungsten oxide polyhedrons were analyzed. The tungsten oxide phases found in the cross section of the W18O49 nanowhisker were analyzed by nanodiffraction and electron energy loss spectroscopy (EELS), which were discussed and compared in light of theoretical calculations based on the density functional theory method. Finally, the knowledge gained from the in situ SEM and TEM experiments was valorized in developing a multigram synthesis of W18O49/a-SiO2 urchin-like nanofibers in a flow reactor.

Preparation of -WO3/a-SiO2 Nanofibers PVA (150 g) was dissolved in deionized water (1150 g) by stirring and heating for several hours providing an 11.5 wt% solution.Silicotungstic acid hydrate (120 g) was dissolved in deionized water (200 g).Both solutions were combined at ambient temperature and homogenized by intensive stirring for several hours.The final polymer and tungsten precursor contents in the prepared solution were approx.9.3 and 7.4 wt%, respectively.Thermogravimetric analysis and differential scanning calorimetry (TG/DSC) were performed on a Netzsch Jupiter STA 449 instrument with a heating rate of 10 K min −1 and a maximum temperature of 1000 °C.
The prepared solution (approx.800 cm 3 ) was transferred to the electrode vessel with a partially submerged electrode.A grounded counter electrode in the form of a stretched wire was covered with a large sheet of aluminum foil (approx.0.8 x 0.4 m), and the electrode distance was set to 17.0 cm.The speed of the rotating electrode was set to 30 rpm.The applied voltage was set to 50 kV.The process was running for 4 h until one half of the solution was consumed, and the aluminum collector was covered by a thick layer of nonwoven felt of green composite of PVA and silicotungstic acid.The prepared fibrous material was peeled off, analyzed by SEM (Figure S2) and TGA/DSC (Figure S3) and used in further process.The green fibers from PVA and HSiW were calcined in air at 600 °C in a muffle furnace.The furnace was heated during 1 h to the final temperature followed by 2 h of dwell time.After heat treatment, the sample was left to cool down spontaneously to ambient temperature.The nanofibrous -WO3/a-SiO2 material was analyzed by X-ray diffraction (XRD) (Figure S4) and SEM (Figure S5).

Kinetic Study of Growth of W18O49 Nanowhiskers in-situ in the SEM
Following Video 1, a kinetic study of the growth of W18O49 nanowhiskers on the surface of an e-beam irradiated -WO3/a-SiO2 nanofiber in the Reactor within SEM was undertaken (Figure S8).Two nanowhiskers (marked A and B), which exhibited some differences in their growth mode are shown (at 800 °C and under 100 Pa of H2 gas).Note that both nanowhiskers changed their lateral size (thickness) while growing in length in the Reactor.The discrepancy between the growth rates in the linear part and the final dimensions of the two W18O49 nanowhiskers is displayed in Figure S8 and in Table S1, respectively.The length-time dependence of the growing nanowhiskers is shown in Figure S8.Both studied nanowhiskers grew initially at approximately a linear rate.The initial growth rate of the 22 nm nanowhisker (A) was three times faster than the 40 nm specimen (B).Bundles were formed by the emergence of neighboring nanowhiskers seeded by the primary nanowhisker. 43However, the resulting morphology of the formed structures differed.The nanowhisker A appears inhomogeneous, with a bundle of nanowhiskers of different lengths grown in the same direction.While some grow with time, others disappear, likely due to Ostwald ripening.
Alternatively, the diminishing of some nanowhiskers could be attributed to the combined effect of the heating and the high vacuum in the chamber, which could pump out the tungsten oxide vapors.The primary nanowhisker could be distinguished in the bundle as the longest (see arrow in Figure S8 4A-8A).On the contrary, bundle B was appreciably more compact and appeared as a single nanowhisker consisting of several domains with well-defined grain boundaries.The multidomain pattern of bundle B is more challenging to observe (in SEM), and only the thickening of the nanowhisker suggests its bundle-like character.Interestingly, both structures underwent shrinking at the last phase of the reaction.The original nanowhisker in bundle A shortened quite significantly after 650 s.On the other hand, bundle B slimmed down and its length shortened only slightly at the end of the in-situ annealing process.
The in-situ growth process of the nanowhisker consists of two counter-acting fluxes.The heated -WO3/a-SiO2 provides a large local concentration of volatile WO3-x clusters, which condense on the W18O49 tip and lead to its rapid growth.On the other hand, the strong vacuum pumping of the SEM and the heating effect of the e-beam lead to evaporation of WO3-x clusters from the nanowhisker tip and its shortening.Initially (100-200 s), the latter effect is insignificant because the massive supply of WO3-x clusters from the root of the -WO3/a-SiO2 nanofiber.However, as the nanowhisker gets longer, this supply diminishes and the pumping/e-beam heating of the nanowhisker leads to evaporation of WO3-x clusters from its tip and shortening.In the meantime, new nanowhiskers start to grow nearby forming a bundle, due likely to this site being a "fertile ground" for such growth.

Figure S1 .
Figure S1.a) Scheme of Nanospider NS LAB500S (Elmarco, Czech Republic) electrospinning setup used for production of PVA-silicotungstic acid nanofibers.b) The cylindrical electrode compartment with micro blades for allocation of electrical charge and solution droplets.42

Figure S2 .
Figure S2.SEM image of green nanofibrous composite of PVA and silicotungstic acid.

Figure S3 .
Figure S3.TG/DSC traces of green composite of PVA and silicotungstic acid measured in air.

Figure S4 .
Figure S4.X-ray powder diffractogram of the -WO 3 /a-SiO 2 precursor nanofibers.The peaks of the WO 3 phase are marked by x.All the peaks of the diffractogram belong to a single WO3 phase (low intensity or signal in close proximity to large peaks were not marked for clarity).

Figure S8 . 1 Figure
Figure S8.Kinetic analysis of growth of selected nanowhiskers.The 22 nm nanowhisker (A) was labeled 1-8A.Sequence 1-8B displays the growth mode of nanowhisker B. The fitting of the initial growth rate is (1-3A) 2.7 nm s −1 .Comparably, the initial growth rate of the nanowhisker B was only 0.9 nm s −1 .

Figure S11 .
Figure S11.In-situ TEM analysis of the W18O49 nanowhisker growth at 820 °C (under e-beam irradiation).The growth rate was linear at 9 nm s −1 .The nanowhisker growth was observed out of focus in order to minimize the influence of the e-beam on the whisker growth.

Figure S12 .
Figure S12.In-situ TEM images of shear plane translation in the structure of the growing W18O49 nanowhisker at 820 °C (under e-beam irradiation).Three shear planes were observed simultaneously, marked red, yellow, and blue.The red shear plane translates along the <010> crystal axis towards the nanowhisker base.On the other hand, the yellow-marked shear plane shifts in the opposite direction simultaneously.Finally, the blue-marked shear plane is stationary.

Figure S16 .
Figure S16.SEM images of -WO3/a-SiO2 nanofibers processed in the tube furnace under vacuum at a) 800 °C and b) 900 °C for one hour.Evidently, 800 °C is not a sufficient temperature for the W18O49 nanowhisker growth (a), which corresponds closely with the in-situ observation in the Reactor within the SEM (see FigureS9, t = 0 s).On the other hand, the temperature of 900 °C (b) is inducing the growth of WO3-x nanowhiskers.

Table S1 .
Comparison between the growth kinetics of two different e-beam irradiated W18O49 nanowhiskers *-growth rate calculated from the linear fit of the length-time dependence